Reactive Working Material for Use in Hydrogen Production by Decompostion of Water

ABSTRACT

Disclosed is a reactive working material for use in a process of producing hydrogen by splitting water based on a two-step thermochemical water-splitting cycle through the utilization of solar heat, industrial waste heat or the like, which comprises a ferrite fine powder and a cubic zirconia supporting the ferrite fine powder. This reactive working material makes it possible to prevent scaling off of the ferrite fine powder from the zirconia fine powder due to volumetric changes of the ferrite fine powder during repeated use, and suppress growth of FeO grains due to repetition of melting and solidification when used as a reactive working material for a cyclic reaction under a high temperature of 1400° C. or more.

TECHNICAL FIELD

The present invention relates to a technique of producing hydrogen bysplitting water through the utilization of solar heat, industrial wasteheat or the like, and more particularly to a hydrogen production processbased on a two-step thermochemical water-splitting cycle and a reactiveworking material for use in the hydrogen production process.

BACKGROUND ART

A hydrogen production process based on a two-step thermochemicalwater-splitting cycle has been widely known before the filing of thispatent application. The hydrogen production process is designed torepeat the following two reaction formulas.MOox→MOred+½O₂   First StepMOred+H₂O→MOox+H₂   Second Step

Specifically, this hydrogen production process comprises a first step ofreducing a metal oxide MOox to form a reduced metal oxide MOred andproduce oxygen through a high-temperature thermal decompositionreaction, and a second step of reacting the reduced metal oxide withwater to oxidize the reduced metal oxide to a metal oxide and producehydrogen.

Typically, magnetite Fe₃O₄, which is known and described as “iron-basedoxide” or “ferrite”, is used as the metal oxide MOox, i.e., a reactiveworking material for the hydrogen production process. This iron-basedoxide as the reactive working material is reduced to wustite FeO in thefirst step to release oxygen, and the wustite FeO is reacted with waterin the second step to release hydrogen and return to magnetite Fe₃O₄.Then, the reactive working material will be reused.

In the above reaction formulas, the process of releasing oxygen in thefirst step is generally required to perform under a high-temperatureatmosphere of 1800 to 2300° C. In practice, under such ahigh-temperature atmosphere for the oxygen release reaction, theiron-based oxide is sintered to be deactivated and cause quite strongvaporization. Therefore, it is required to quench the vaporizedsubstance, and this requirement makes it difficult to put the two-stepthermochemical water-splitting cycle to practical use.

With a view to solve the problem concerning the reactive workingmaterial for use in the hydrogen production process based on thetwo-step thermochemical water-splitting cycle, the applicant of thispatent application previously disclosed a hydrogen production-processbased on a two-step thermochemical water-splitting cycle which uses areactive working material comprising ferrite fine powder and zirconiafine powder supporting the ferrite fine powder, in Japanese PatentApplication No. 2003-060101.

This reactive working material is formed such that ferrite fine powderis supported on zirconia fine powder. The zirconia fine powder is hardlysintered even at high temperatures, and the ferrite fine particlessupported on the zirconia fine powder is prevented from coming in closecontact with other ferrite particles each other so as to suppress graingrowth thereof to provide enhanced reactivity and reusability even at arelatively low temperature of 1400° C. or less.

Fe₃O₄ and FeO to be repeatedly formed during the reaction cycle havespecific gravities of 5.2 and 5.7, respectively. Thus, due to volumetricchanges of these powders during the reaction cycle, the ferrite finepowder will scale off from the zirconia fine powder to spoil thezirconia powder's effect of suppressing grain growth in the ferrite finepowder., Moreover, under a reaction atmosphere repeatedly having atemperature of 1400° C. or more which is greater than a melting point ofFeO, the ferrite fine powder will be gradually agglomerated to causegrain growth while repeating melting and solidification, resulting indeterioration of reaction efficiency.

DISCLOSURE OF THE INVENTION

In a reactive working material for use in a hydrogen production processbased on a two-step thermochemical water-splitting cycle, whichcomprises ferrite and zirconia supporting the ferrite, it is an objectof the present invention to provide an effective means for preventingthe ferrite from scaling off the zirconia due to volumetric changes ofthe ferrite during repeated use.

It is another object of the present invention to provide a means forsuppressing growth of FeO grains due to repetition of melting andsolidification when used as a reactive working material for a cyclicalreaction under a high temperature of 1400° C. or more.

In order to achieve the above objects, the present invention provides areactive working material for use in a two-step thermochemicalwater-splitting cycle, which comprises ferrite and zirconia supportingthe ferrite, wherein the zirconia supporting ferrite is a cubiczirconia.

As used in this specification, the “ferrite” means an oxide representedby a composition formula of M(II)O.Fe₂O₃, wherein M(II) is a divalentmetal, such as Fe, Mn, Co, Mg, Ni, Zn or Cu. The oxide constituting theferrite may have any configuration. For example, Fe₃O₄ having a spinelcrystal structure may be used. The divalent metals, such as Mn, Co orMg, may be effectively doped as ions by replacing ferrous ion in Fe₃O₄partially or all. When used in the form of a fine powder, the ferrite isprepared to have a particle size, preferably, of 10 μm or less, morepreferably 1 μm or less.

Further, the “cubic zirconia” means a fully-stabilized zirconia or apartially-stabilized zirconia which contains a stabilizer, such ascalcia or yttria, and a zirconia including a cubic crystal phase.Preferably, the cubic zirconia contains yttria or calcia in an amount of2 mol % or more. If the content rate of the stabilizer is less than 2mol %, the suppression of grain growth in the ferrite will becomeinsufficient. An excessive content rate of the stabilizer causesdeterioration in reactivity. Thus, more preferably, an upper limit ofthe content rate is set at 25 mol % or less.

In the present invention, the reactive working material comprising aferrite and a zirconia supporting the ferrite may be formed as aferrite/zirconia composite powder. Alternatively, the reactive workingmaterial may be formed as a ferrite-supporting porous zirconia ceramics.In this case, a ferrite fine powder may be coated on, i.e., supportedon, a porous structure of a porous zirconia ceramics.

The ferrite/zirconia composite powder may be prepared by the followingspecific method.

As one example, a method using an aqueous Fe(II) salt solution may beemployed. Specifically, an yttria fully-stabilized or partiallystabilized zirconia fine powder or a calcia fully-stabilized orpartially stabilized zirconia fine powder which has a particle size of10 μm or less, preferably 1 μm or less, is dispersed in an aqueousFe(II) salt solution or an aqueous Fe(II) salt solution containinganother metal salt dissolved therein as a doping metal [M(II)], and anaqueous alkali hydroxide solution is added to the zirconia finepowder-dispersed aqueous solution to form a Fe (II) hydroxide colloidtherein. Then, air is bubbled in the colloid-containing aqueous solutionto oxidize the Fe (II) hydroxide colloid. Then, adissolution-precipitation reaction where the Fe (II) hydroxide colloidis dissolved in the zirconia fine powder-dispersed aqueous solution andthen precipitated as a ferrite is promoted to grow Fe₃O₄ orM_(x)Fe_(3-x)O₄ on the dispersed zirconia fine powder so as to form aferrite/zirconia composite powder.

The porous zirconia ceramics supporting the ferrite may be prepared byimmersing a porous zirconia ceramics including a cubic crystal phaseinto the above aqueous Fe(II) salt solution or the above aqueous Fe(II)salt solution containing another metal salt dissolved therein as adoping metal [M(II)], drying the pulled-out porous zirconia ceramicsbody, and subjecting the dried porous zirconia ceramics to a heattreatment. In the same manner, a reactive working material having aferrite fine powder coating can be prepared using a cubic zirconiahaving any other configuration.

As another method for preparing the ferrite/zirconia composite powder, asolvent impregnation process may be used. Specifically, afully-stabilized or partially-stabilized zirconia fine powder isdispersed in an aqueous solution of a metal salt, such as iron nitrate,iron chloride or organic iron, and a salt of the doping metal. Theobtained mixture is evaporated and dried, and then the dried mixture isburnt to allow the metal salt on the zirconia to be decomposed to themetal oxide. Then, the metal oxide is heated under a H₂/H₂ 0 mixed gasatmosphere or a CO/CO₂ mixed gas atmosphere at a temperature of 300° C.or more.

The porous zirconia ceramics supporting the ferrite may be prepared byimmersing a porous zirconia ceramics including a cubic crystal phaseinto the above aqueous Fe(II) salt solution or the above aqueous Fe(II)salt solution containing another metal salt dissolved therein as adoping metal [M(II)], drying the pulled-out porous zirconia ceramicsbody, and subjecting the dried porous zirconia ceramics to a heattreatment. In the same manner, a reactive working material having aferrite fine powder coating can be prepared using a cubic zirconiahaving any other configuration.

When a reaction temperature is increased up to 1300 to 1500° C., theferrite fine powder supported on the zirconia is formed as FeO throughrelease of oxygen therefrom. Subsequently, when the reaction temperatureis decreased to 1000° C. and water vapor is introduced, FeO returns tothe original Fe₃O₄ through oxidization while decomposing water togenerate hydrogen.

In the above process using the fully-stabilized cubic zirconia, duringthe course of the formation of FeO at the high temperatures, FeO isincorporated into the zirconia as a solid solution to form a cubiczirconia containing iron ions in the zirconia lattice. In this case,even during the course of the oxidization to Fe₃O₄ as well as that ofthe thermal reduction of Fe₃O₄, it is impossible that FeO particlessupported on the zirconia fine power scale off the zirconia support andexist as independent grains since FeO phase is not formed anymore as thereduced iron-based oxide.

In the partially-stabilized cubic zirconia, while tetragonal andmonoclinic crystal phases exist as a zirconia crystal together with acubic crystal phase, the zirconia formed as a solid solution with FeOstabilizes the cubic crystal phase to allow the entire crystal phase tohave a transition to a cubic zirconia.

As above, at a high reaction temperature of 1300 to 1500° C., Fe₃O₄ andcubic zirconia are changed to FeO-zirconia solid solutions throughrelease of oxygen therefrom. Then, when the reaction temperature isreduced to 1000° C. and water vapor is introduced, iron ions in thesolid solution of the cubic stabilized zirconia are oxidized whilegenerating hydrogen, and a Fe₃O₄ fine powder is precipitated on thecubic zirconia.

That is, in the hydrogen production process based on the two-stepthermochemical water-splitting cycle, a first step where a solidsolution of FeO and cubic zirconia is formed at high temperatures, and asecond step where a Fe₃O₄ fine powder is precipitated from the solidsolution of cubic zirconia incorporating iron ions and formed directlyon the cubic zirconia during decomposition of water at low temperatures,will be repeatedly performed.

A temperature allowing the cubic solid solution of FeO and zirconia tobe fully molten is 2000° C. or more when a ratio of FeO to the solidsolution is 30 wt % or less. Thus, the solid grains are not excessivelymolten during the cyclic reaction, and grain growth is suppressed. Thatis, FeO is not changed to independent grains, and therefore anundesirable situation where FeO grains are molten and agglomerated atabout 1400° C., i.e., a melting point thereof, can be avoided tosuppress grain growth.

In addition, FeO incorporated in the zirconia as a solid solution ispenetrated into and strongly bonded with the zirconia crystal toeliminate the problem about scaling off of the ferrite fine grains fromthe zirconia fine grains due to volumetric changes, i.e., a largedifference in specific gravity between Fe₃O₄ as a reactive workingmaterial and FeO to be formed through the reaction.

Thus, a fully-stabilized or partially-stabilized cubic zirconia used asa zirconia fine powder for supporting a ferrite fine powder makes itpossible to eliminate the problem about scaling off of the ferrite finepowder from the zirconia fine powder as in a case of using a monocliniczirconia as a support, and achieve an effect of preventing excessivesintering of the ferrite fine powder.

In the present invention, a hydrogen production process using theferrite supported on the fully-stabilized or partially-stabilizedzirconia is performed according to the following reaction formulas.

In the use of an yttria fully-stabilized or partially-stabilizedzirconia: $\begin{matrix}\begin{matrix}{{{x/3}{Fe}_{3}O_{4}} + {Y_{y}{Zr}_{1 - y}{O_{2 - {y/2}}\left( {{{cubic}\quad{crystal}\quad{phase}},} \right.}}} \\\left. {{{or}\quad{cubic}\quad{crystal}\quad{phase}} + {{tetragonal}\quad{crystal}\quad{phase}}} \right) \\{{= {{{Fe}_{x}Y_{y}{Zr}_{1 - y}{O_{2 - {y/2} + x}\left( {{cubic}\quad{crystal}\quad{phase}} \right)}} + {{x/6}O_{2}}}};} \\{and}\end{matrix} & {{Formula}\quad(1)} \\\begin{matrix}{{{Fe}_{x}Y_{y}{Zr}_{1 - y}{O_{2 - {y/2} + x}\left( {{cubic}\quad{crystal}\quad{phase}} \right)}} + {{x/3}H_{2}O}} \\{= {{{x/3}{Fe}_{3}O_{4}} + {Y_{y}{Zr}_{1 - y}{O_{2 - {y/2}}\left( {{cubic}\quad{crystal}\quad{phase}} \right)}} + {{x/3}H_{2}}}}\end{matrix} & {{Formula}\quad(2)}\end{matrix}$

In the use of a calcia fully-stabilized or partially-stabilizedzirconia: $\begin{matrix}\begin{matrix}{{{x/3}{Fe}_{3}O_{4}} + {{Ca}_{y}{Zr}_{1 - y}{O_{2 - y}\left( {{{cubic}\quad{crystal}\quad{phase}},} \right.}}} \\{{{or}\quad a\quad{combination}\quad{of}\quad{cubic}\quad{crystal}\quad{phase}} +} \\\left. {{{monoclinic}\quad{crystal}\quad{phase}} + {{tetragonal}\quad{crystal}\quad{phase}}} \right) \\{{= {{{Fe}_{x}{Ca}_{y}{Zr}_{1 - y}{O_{2 - y + x}\left( {{cubic}\quad{crystal}\quad{phase}} \right)}} + {{x/6}O_{2}}}};} \\{and}\end{matrix} & {{Formula}\quad(3)} \\{\quad{{{Fe}_{x}{Ca}_{y}{Zr}_{1 - y}{O_{2 - y + x}\left( {{cubic}\quad{crystal}\quad{phase}} \right)}} + {{x/3}H_{2}O}}} & {{Formula}\quad(4)} \\{= {{{x/3}{Fe}_{3}O_{4}} + {{Ca}_{y}{Zr}_{1 - y}{O_{2 - y}\left( {{cubic}\quad{crystal}\quad{phase}} \right)}} + {{x/3}H_{2}}}} & \quad\end{matrix}$

The present invention provides the following advantages.

A crystal growth of the iron-based oxide as the reactive workingmaterial can be suppressed to maintain cyclical reactivity.

The oxygen-releasing temperature is reduced in the second reaction stepsince the grain growth of the iron oxide particles is effectivelysuppressed. This lower reaction temperature makes it possible toeliminate the need for quenching of the reactive working material and alarge-scale facility for the quenching.

Thus, a process of converting natural heat energy, such as solar heat,to chemical energy of hydrogen can be achieved.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described based onthe following Example.

EXAMPLE 1

An yttria partially-stabilized zirconia (YSZ) represented by(ZrO₂)_(0.97) (Y₂O₃)_(0.03) and a calcia partially-stabilized zirconia(CSZ) represented by (ZrO₂)_(0.97) (CaO)_(0.03) (produced by KojundoChemical Lab. Co. Ltd.) were used as a support of a ferrite. Each of theYSZ and CSZ has a particle size of 1 μm or less. The YSZ primarilycomprises a cubic crystal phase and slightly has a tetragonal crystalphase. The YSZ has a BET (Brunauer-Emmett-Teller) surface area of 7.7m²g⁻¹. The CSZ primarily comprises a cubic crystal phase and partiallyhas a monoclinic crystal phase. As Comparative Example, a conventionalmonoclinic zirconia (BET surface area: 12.6 m²g⁻¹) was used as asupport.

A cubic zirconia-supported ferrite as Inventive Example and a monocliniczirconia-supported ferrite as Comparative Example were prepared throughthe following process.

The zirconia particles were suspended in distilled water after removingoxygen and CO₂ therefrom, and N₂ was supplied therethrough for 1 hour toexpel any remaining air. Then, a given amount of FeSO₄, MnSO₄ and CoSO₄were dissolved in the suspension, and 0.15 mol dm⁻³ of NaOH solution wasadded to the obtained solution to adjust pH at 8.5 so as to formhydroxides of Fe²⁺, Mn²⁺ and Co²⁺. A mass ratio of a ferrite phase tothe zirconia is set in the range of 20 to 25%. This mixed solution washeated up to 65° C., and then an NaOH solution was added to the heatedsolution while supplying air bubbles therethrough, to maintain pH at8.5. A resulting product was collected by centrifugal separation at14000 rpm. The collected product was rinsed with distilled water andacetone, and then dried at room temperature all day and night. Inadvance of a high-temperature reaction, the sample prepared in the abovemanner was subjected to a high-temperature stabilization treatment underan N₂ atmosphere at 900° C. Additionally, as Comparative Example, Fe₃O₄without a zirconia support was prepared through a coprecipitationprocess.

The obtained samples were identified using an X-ray diffractometer (XRD)(RAD-γ A diffractometer: produced by Rigaku Co. Ltd.). A ferrite phasein each of the samples was dissolved by HCl, and respective amounts ofFe, Mn and Co in the solution were analyzed through inductively coupledplasma (ICP) emission spectrometry (SPS-1500 V: SEIKO Instruments Inc.)to determine a weight ratio of ferrite/zirconia support.

Each of the samples was used in the water-decomposition reactionrepresented by the aforementioned formulas (1) and (2) or (3) and (4).The reaction represented by the formula (1) or (3) and the reactionrepresented by the formula (2) or (4) will hereinafter referred torespectively as “thermal reduction reaction” and “water-decompositionreaction”.

FIG. 1 shows an experimental apparatus for the thermal reductionreaction, which is designed to supply N₂ gas to a silica tube 4 which iscontained in an infrared furnace 1 to house a platinum cup 2 forreceiving the solid sample 3 therein, and exhaust the N₂ gas from thesilica tube 4.

The solid sample (about 1 g) 3 was put in the platinum cup (diameter: 10mm, depth: 7 mm) 2, and set in the silica tube (SSA-E45; produced byULVAC-RIKO, Inc) 4 having an inner diameter of 45 mm. N₂ gas (purity:99.999%) was supplied at a flow rate of 1.0 Ndm³·min. A temperature ofthe platinum cup 2 was controlled using an R-type thermocouple 5disposed in contact with the platinum cup 2 in the infrared furnace(RHL-VHT-E44: produced by ULVAC-RIKO, Inc) 1, and increased up to agiven temperature (1400° C.) so as to induce the thermal reductionreaction.

The solid sample 3 was heated at a constant temperature for 0.5 hours,and then cooled to room temperature. After completion of the reaction,the thermally reduced solid sample 3 was crushed using a mortar, and theobtained powder was put in the silica tube 4. Then, H₂O/N₂ mixed gas wasintroduced into the reaction system to induce the water-decompositionreaction. The H₂O/N₂ mixed gas was supplied at a flow rate of 4Ncm³·min⁻¹ through distilled water having a temperature of 80° C. Inthis process, a partial pressure of water vapor in the H₂O/N₂ mixed gaswas estimated as 47% from a vapor pressure at 80° C. and 1 atm.

The reaction system was heated up to 1000° C. within 10 minutes using aninfrared furnace (RHL-E45P: produced by ULVAC-RIKO, Inc) 1, and then thewater-decomposition reaction was promoted at a constant temperature of1000° C. for 50 minutes.

In order to measure an amount of hydrogen generated through thewater-decomposition reaction, the discharged gas was collected overwater to a container. After completion of the water-decompositionreaction, a volume of the collected discharged gas was measured, and gascomponents were determined using a thermal conductivity detector (TCD)gas chromatography (GC-4C: produced by Shimadzu Corp.). Each of thesamples was identified using the XRD (RAD-γ A diffractometer: producedby Rigaku Co. Ltd.).

Further, in order to check repeatability of the cyclical reaction, thethermal reduction reaction and the water-decomposition reaction usingthe ferrite/zirconia composite powder were alternately repeated two toseven times. After completion of each cycle, the thermally reducedsample was crushed using a mortar.

FIGS. 2(a) and 2(b) show respective XRD patterns of the YSZ supporting20 wt % of Fe₃O₄ in two reaction stages, wherein FIG. 2(a) shows an XRDpattern before initiation of the reaction, and FIG. 2(b) shows an XRDpattern after completer of the high-temperature thermal reductionreaction at 1400° C.

As shown in FIGS. 2(a) and 2(b), a strong peak of a cubic ZrO₂ wasobserved together of a peak due to the reflection of Fe₃O₄. Further, aslight tetragonal crystal phase was observed in a part of the ZrO₂ phaseof the YSZ.

For comparison, this YSZ-supported Fe₃O₄ and Fe₃O₄ itself without theYSZ support were thermally reduced at 1400° C.

After the thermal reduction, the YSZ-supported Fe₃O₄ was formed asporous pellets in the platinum cap. This pellet was easily crushed usingthe mortar, which was significantly effective in using for generatinghydrogen in the subsequent water-decomposition reaction. In contract,the Fe₃O₄ without the YSZ support was severely sintered in the platinumcap, and formed as a dense, hard, candy-like glossy agglomerate. Theagglomerate was attached onto a bottom surface of the platinum cap. Itwas assumed that the sample is molted once at the high temperature andthen cooled and solidified again. The reason is that the thermalreduction reaction temperature of 1400° C. is close to a melting point(1370° C.) of the reduced Fe₃O₄ phase. This agglomerate was hardlycrushed using the mortar, and therefore the subsequentwater-decomposition reaction could not be performed using the Fe₃O₄without the YSZ support. These test results show that the YSZ supportcan effectively suppress sintering of an iron oxide at hightemperatures.

FIG. 3 shows a hydrogen generation rate per gram of the sample and awater-decomposition temperature in the 1st cycle of thewater-decomposition reaction, which represents a hydrogen generationprofile in the water-decomposition reaction using the YSZ supportingFe₃O₄ (20 wt %) thermally reduced at a reaction temperature of 1400° C.by plotting a hydrogen generation rate (Ncm³·min⁻¹per gram of the samplewith respect to a reaction time.

Further, the two-step water-splitting reaction using the Fe₃O₄/YSZreactive working material was repeated seven times to checkrepeatability of the cyclical reaction.

FIG. 4 shows a measurement result of a hydrogen generation amount pergram of the reactive working material sample in each cycle. Therepeatability was checked using two samples having a Fe₃O₄ weight ratioof 20 wt % and 25 wt %. In FIG. 4, the sample having 20 wt % of Fe₃O₄and the sample having 25 wt % of Fe₃O₄ are indicated by a black circleand a while circle, respectively. As seen in FIG. 4, a hydrogengeneration amount in each of the samples is maintained at anapproximately constant value even if the number of cycles is increased.Further, a hydrogen generation amount is drastically increased in mostof the cycles by increasing the Fe₃O₄ weight ratio from 20 wt % to 25 wt%.

A rate of Fe³⁺ in Fe₃O₄ which is reduced to Fe²⁺ in the thermalreduction reaction will hereinafter be referred to as “Fe₃O₄ conversionrate”. Then, the Fe₃O₄ conversion rate was roughly estimated on theassumption that “Fe²⁺ generated through the thermal reduction reactionwill be entirely re-oxidized to Fe³⁺ through the water-decompositionreaction”.

Table 1 shows a Fe₃O₄ conversion rate in each of the seven cycles of thethermal reduction reaction, according to the above rough estimation. Asseen in Table 1, when 20 wt % of Fe₃O₄ is supported, 20 to 30% of theFe₃O₄ phase is converted to a reduced phase. Further, when the contentrate of Fe₃O₄ is increased to 25 wt %, the Fe₃O₄ conversion rate isincreased to about 30 to 40%.

In Table 2, given that a sample having Fe₃O₄ supported on the YSZ isexpressed as Fe₃O₄/YSZ; a sample having Co or Mn-doped Fe₃O₄ supportedon the YSZ is expressed as Co_(x)Fe_(3-x) O₄/YSZ or Mn_(x)Fe_(3-x) 0₄/YSZ; a sample having Fe₃O₄ supported on the CSZ is expressed asFe₃O₄/CSZ; and a sample having Fe₃O₄ supported on the monocliniczirconia is expressed as Fe₃O₄/monoclinic zirconia, respective hydrogengeneration amounts (Ncm³ g⁻¹) in these samples are compared with eachother. While hydrogen is continuously generated in each of the samples,the hydrogen generation amount in the Fe₃O₄/monoclinic zirconia issharply reduced in and after the 5th cycle. In contract, such a decreasein the hydrogen generation amount is not observed in the samples havingthe cubic zirconia support, i.e., the Fe₃O₄/YSZ and the Fe₃O₄/CSZ. Thereason would be that grain growth due to the high-temperature reactioncycles is suppressed in the cubic zirconia-supported ferrite as comparedwith the monoclinic zirconia-supported ferrite.

FIGS. 5(a) to 5(d) show an SEM image (×1000) of the Fe₃O₄/YSZ andelement mappings of the sample after the 7th cycle using an electronprobe microanalyzer (EPMA), wherein FIG. 5(a) shows the sample beforethe reaction, and FIG. 5(b) shows the sample after the 7th cycle. FIG.5(c) is a photograph showing a Fe distribution, and FIG. 5(d) aphotograph showing a Zr distribution.

FIGS. 6(a) to 6(d) show an SEM image of the Fe₃O₄/monoclinic zirconiaand Fe and Zr distributions after the 6th cycle using the EPMA.

As is clear from the comparison between the SEM images in these figures,while a Fe₃O₄ grain in the Fe₃O₄/monoclinic zirconia grows to have asize equivalent to that of a ZrO₂ grain in the 6th cycle, such a largegrain growth is not observed in the Fe₃O₄/YSZ even after the 7th cycle,and the Fe₃O₄ grain has a size of 2.5 μm or less. Further, it can beobserved that there are many overlapped regions between the ZrO₂ andFe₃O₄ distributions. This shows that a Fe₃O₄ fine powder is precipitatedon a surface of the YSZ.

It is believed that the above result could be obtained by the followingreason. Differently from the Fe₃O₄/monoclinic ZrO₂, in the Fe₃O₄/YSZ, areduced ion is incorporated in a crystal structure of the YSZ during thethermal reduction reaction, instead of being formed as a FeO grain, andthen precipitated as a Fe₃O₄ grain from the zirconia during thewater-decomposition reaction, so that aggregation between FeO grains dueto melting/solidification can be avoided.

FIGS. 7(a) to 7(c) show a peak due to the reflection of a Fe₃O₄ (311)face in an XRD pattern of the Fe₃O₄ (20 wt %)/YSZ during thewater-decomposition reaction in each cycle, wherein: FIG. 7(a) shows anXRD pattern before the reaction; FIG. 7(b) shows an XRD 3pattern afterthe high-temperature thermal reduction reaction at 1400° C.; and FIG.7(c) shows an XRD pattern after the water-decomposition reaction.

As shown in FIGS. 7(a) and 7(b), in the XRD pattern of the Fe₃O₄/YSZafter the thermal reduction reaction, the peak originated from the Fe₃O₄(311) face becomes weaker than that of the Fe₃O₄/YSZ before thereaction. Further, after the thermal reduction reaction, no peakoriginated from the FeO is observed in the XRD pattern, and a weak peakof a tetragonal ZrO₂ disappears. That is, as seen in FIG. 2(b), in XRDpattern after the thermal reduction reaction, only a strong peak ofcubic ZrO₂ and a weak peak of Fe₃O₄ are observed. Then, after thewater-decomposition reaction, as seen in FIG. 7(c), the intensity of apeak originated from Fe₃O₄ is increased again.

As evidenced by the above result, during the thermal reduction reaction,the Fe₃O₄ reacts with the YSZ through release of oxygen, i.e., Fe⁺²penetrates into a cubic YSZ lattice to form a cubic ZrO₂ phase includingFe⁺² or Fe⁺² ion is incorporated in a cubic YSZ lattice, as is commonlyknown. Then, during the water-decomposition reaction, theFe⁺²-containing YSZ generates hydrogen through decomposition of water,to form a Fe₃O₄ phase on the YSZ support. This cyclical reaction may beroughly represented by the aforementioned formulas (1) and (2). It wasalso verified that a sample using CSZ has the same reaction mechanismrepresented by the aforementioned formulas (3) and (4). TABLE 1 FerriteRatio/ Fe₃O₄ Conversion Rate/% wt %-Fe₃O₄ 1st 2nd 3rd 4th 5th 6th 7th 2027 27 29 29 25 17 25 25 34 18 28 36 33 38 34

TABLE 2 Temperature of Thermal Hydrogen Generation Reactive workingReduction Amount/Ncm³g⁻¹ material Reaction/° C. 1st 2nd 3rd 4th 5th 6th7th Fe₃O₄ 1400 6.9 5.5 7.9 6.6 5.4 6.7 7.8 (20.3 wt %)/YSZ Fe₃O₄ 14008.9 4.6 7.1 9.2 8.4 9.9 8.9 (24.5 wt %)/YSZ Co_(0.39)Fe_(2.61)O₄ 14006.4 2.9 8.3 5.4 8.4 7.5 5.4 (19.5 wt %)/YSZ CO_(0.68)Fe_(2.32)O₄ 14006.5 5.1 6.0 7.3 6.3 6.2 5.1 (18.3 wt %)/YSZ Mn_(0.68)F_(2.62)O₄ 1400 8.03.3 5.7 3.4 — — — (16.1 wt %)/YSZ Fe₃O₄ 1400 5.5 6.2 6.7 5.3 5.7 7.2 6.6(23.4 wt %)/CSZ CO_(0.38)Fe_(2.62)O₄ 1400 2.9 3.7 6.5 6.9 5.2 — — (18.4wt %)/CSZ Fe₃O₄ 1400 7.7 7.2 7.4 6.0 4.7 4.2 — (20.0 wt %)/ monoclinicZrO₂

INDUSTRIAL APPLICABILITY

The applicant of this patent application previously disclosed anactivity of a two-step water-splitting cycle using an iron-based oxide,i.e., ferrite, supported on a zirconia, in Japanese Patent ApplicationNo. 2003-060101. Specifically, it was disclosed that the ferritesupported on the zirconia can suppress aggregation and sintering, andthereby allows a two-step water-decomposition reaction to be repeatedlyperformed in a temperature cycle of 1000° C. and 1400° C. with enhancedreactivity as compared with a ferrite without a zirconia support. It wasalso disclosed that the occurrence of a phase change between Fe₃O₄ andFeO on a surface of the zirconia along with oxidation/reductionreactions of a solid phase is verified using an XRD.

The present invention is based on knowledge that a transition from Fe³⁺to Fe²⁺ represented by the aforementioned formulas (1) and (2) or theaforementioned formulas (3) and (4) is observed in a ferrite supportedon a cubic zirconia without formation of FeO grain.

In this reaction system, it is believed that Fe²⁺ generated through thethermal reduction reaction penetrates into and exists in the cubiczirconia lattice instead of being formed as a FeO crystal, and thereforethe melting of the FeO phase is suppressed during the thermal reductionreaction. Thus, in the cubic zirconia-supported ferrite, themelting/aggregation of the FeO phase at 1400° C. can be suppressed.

Further, as compared with a non-stabilized zirconia-supported ferrite, afully-stabilized or partially stabilized zirconia-supported ferrite ismore suitable for a cyclical reaction involving a temperature change ina high-temperature range. The reason is that, while the non-stabilizedzirconia is changed from a monoclinic crystal phase to a tetragonalcrystal phase due to such a temperature change, iron ions incorporatedin the fully-stabilized or partially stabilized zirconia as a solidsolution can stabilize a cubic crystal phase thereof to suppress atransition in the crystal phase due to the temperature change. When thiszirconia-supported ferrite is fixed onto a ceramic foam to form areaction device, a junction with the ceramic form is likely to havecracks due to volumetric changes caused by a transition in the crystalphase, and consequently scaling off of the zirconia-supported ferrite.It is expected that the fully-stabilized or partially stabilizedzirconia can solve such a problem.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a high-temperature thermalreduction reactor used in a test on a reactive working materialaccording to the present invention.

FIGS. 2(a) and 2(b) are graphs showing an X-ray diffraction (XRD)pattern in each reaction step.

FIG. 3 is a graph showing a water-decomposition temperature and ahydrogen generation rate per gram of a reactive working material in theI st cycle of a water-decomposition reaction.

FIG. 4 is a graph showing a hydrogen generation amount per gram of thereactive working material in each cycle.

FIGS. 5(a) to 5(d) are scanning electron microscopic (SEM) images of aninitial reactive working material and the reactive working materialafter the 7th cycle.

FIGS. 6(a) to 6(d) are SEM images of a monoclinic zirconia-supportedreactive working material.

FIGS. 7(a) to 7(c) are graphs showing changes in Fe₃O₄ (311) face basedon an XRD pattern in each reaction step

DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS

-   1: infrared furnace-   2: platinum cup-   3: solid sample-   4: silica tube-   5: R-type thermocouple

1. A reactive working material for use in a two-step thermochemicalwater-splitting cycle, said reactive working material comprising aferrite and a zirconia supporting said ferrite, wherein saidferrite-supporting zirconia is a cubic zirconia.
 2. The reactive workingmaterial as defined in claim 1, wherein said cubic zirconia containsyttria or calcia in an amount of 2 mol % or more.
 3. The reactiveworking material as defined in claim 1, wherein said ferrite-supportingzirconia is a composite powder of a ferrite fine powder and a zirconiafine powder.
 4. The reactive working material as defined in claim 1,wherein said ferrite-supporting zirconia comprises a zirconia porousceramics body having a porous structure coated with a ferrite finepowder.
 5. A method of preparing a reactive working material for use ina two-step thermochemical water-splitting cycle, which comprises aferrite and a zirconia supporting said ferrite, said method comprising:dispersing a fully-stabilized or partially-stabilized zirconia finepowder having a particle size of 10 μm or less, in an aqueous Fe (II)salt solution, and adding an aqueous alkali hydroxide solution to saidzirconia fine powder-dispersed aqueous solution to form a Fe (II)hydroxide colloid therein; bubbling air in said colloid-containingaqueous solution to oxidize the Fe (II) hydroxide colloid; and promotinga dissolution-precipitation reaction where the Fe (II) hydroxide colloidis dissolved in the zirconia fine powder-dispersed aqueous solution andthen precipitated as Fe₃O₄, so as to grow Fe₃O₄ on the dispersedzirconia fine powder.
 6. A method of preparing a reactive workingmaterial for use in a two-step thermochemical water-splitting cycle,which comprises a ferrite and a zirconia supporting said ferrite, saidmethod comprising: dispersing a fully-stabilized or partially-stabilizedzirconia fine powder in an aqueous solution of a metal salt of Fe (II);evaporating and drying the mixture; and burning the dried mixture toallow the metal salt on the zirconia to be decomposed to the metaloxide; and heating the metal oxide at a temperature of 300° C. or more.7. A hydrogen production process based on a two-step thermochemicalwater-splitting cycle using a reactive working material which comprisesa ferrite and an yttria fully-stabilized or partially-stabilizedzirconia supporting said ferrite, said process comprising two stepsexpressed by the following reaction formulas:x/3Fe₃O₄+Y_(y)Zr_(1−y)O_(2−y/2)=Fe_(x)Y_(y)Zr_(1−y)O_(2−y/2+x)+x/6O₂;andFe_(x)Y_(y)Zr_(1−y)O_(2−y/2+x)+x/3H₂O=x/3Fe₃O₄+Y_(y)Zr_(1−y)O_(2−y/2)+x/3H₂8. A hydrogen production process based on a two-step thermochemicalwater-splitting cycle using a reactive working material which comprisesa ferrite and a calcia fully-stabilized or partially-stabilized zirconiasupporting said ferrite, said process comprising two steps expressed bythe following reaction formulas:x/3Fe₃O₄+Ca_(y)Zr_(1−y)O_(2−y)=Fe_(x)Ca_(y)Zr_(1−y)O_(2−y+x)+x/6O₂; andFe_(x)Ca_(y)Zr_(1−y)O_(2−y+x)+x/3H₂O=x/3Fe₃O₄+Ca_(y)Zr_(1−y)O_(2−y)+x/3H₂